This invention relates to an adhesive for multiple-wire wiring boards using insulated encapsulated metal wires as circuit conductors, and to a multiple wire wiring board using said adhesive and a process for producing the same.
Multiple-wire wiring boards produced by forming an adhesive layer on a substrate, wiring insulated encapsulated wires for forming conductor circuits on the adhesive layer, fixing the encapsulated wires, and connecting circuits between individual layers by through holes are disclosed, for example, in U.S. Pat. Nos. 4,097,684; 3,646,572; 3,674,914 and 3,674,602. These printed wiring boards are known to be advantageous in obtaining high density wiring, matching characteristic impedance and reducing crosstalk.
Ordinary multiple-wire wiring boards are produced, for example as described in the above-mentioned U.S. patents, by forming an adhesive layer comprising a thermosetting resin, a curing agent and rubber on an insulating substrate, fixing insulated encapsulated wires in the adhesive layer, laminating a prepreg or the like, fixing the insulated encapsulated wires on the substrate, cutting insulated encapsulated wires at the portions necessary for connection, drilling holes through the substrate, and metallizing inner walls of the holes. In such processes, by fixing the insulated encapsulated wires in the substrate by means of lamination of the prepreg or the like, it is possible to prevent peeling of the insulated encapsulated wires at the time of drilling, and it is also possible to prevent lowering in reliability due to damage of the coating layer of insulated encapsulated wires at the plating step for forming a metal layer on the inner walls of holes after the step of drilling. Further, reasons for using the adhesive comprising a thermosetting resin, a curing agent and rubber as major components are to make it possible to form a film of an adhesive layer so as to make the handling during the production process easy, since the adhesive layer is prepared by coating an adhesive on a supporting film, followed by drying to form an adhesive sheet and is used by laminating on the insulating substrate or inner circuit boards after lamination of prepregs thereon, followed by lamination and adhesion; to have flexibility and to make it necessary to be non-adhesive except for the step of wiring. In addition, when wires are fixed on the adhesive layer, it is necessary to use an adhesive having a composition which can be activated by thermal energy produced by supersonic vibration, since the wires are contacted with the adhesive by a point of stylus which is vibrated by supersonics.
Recently, printed wiring boards including multiple-wire wiring boards are becoming to have higher multi-layers and much finer in order to attain higher density mounting. When higher multi-layers and finer wiring are applied to the multiple-wire wiring boards, it is very important to maintain certain insulation resistance between wires or between wires and inner layer circuits and positional precision of wires. That is, it is necessary to hold the insulation resistance between neighboring conductors high, and to prevent moving of wires at the step of wiring or after wiring, and the like. In a prior art technique, the insulation resistance was within the working errors so far as the wiring density was belong to that of prior art. As to the positional precision of wires, there was a moving of about 0.2 mm against the designed value after wiring and laminating adhesion of prepreg (hereinafter referred to as "wire swimming"), but since the wiring density was small, such a moving does not prevent the wiring boards from practical use. But with an increase of the wiring density as mentioned above, such a working error becomes to cause remarkable lowering in the insulation resistance. Further, when the positional precision of wires is as mentioned above, wires which should not be connected move to the positions to become through holes. As a result, such wires are cut at the time of drilling. When the inner walls of holes are metallized, there arises a problem in that such wires are connected to circuits other than the desired circuit conductors.
Reasons for lowering the insulation resistance and lowering the positional precision of wires are the use rubber as an adhesive. That is, the insulation resistance of rubber per ser is low, and after wiring, a prepreg or the like is laminated and adhered while retaining the fluidity of the adhesive.
In order to solve such a problem, there has been developed an adhesive comprising a phenoxy resin, an epoxy resin, a curing agent, a reactive diluent and a catalyst for electroless plating as an adhesive sheet as disclosed in Japanese Patent Examined Publication No. 2-12995. That is, by introducing a polymer component having high insulation resistance in place of the rubber of the adhesive mentioned above, the lowering of insulation resistance is suppressed.
On the other hand, the use of an adhesive using a photo initiator in the production of multiple-wire wiring board is known as mentioned below.
Japanese Patent Unexamined Publication No. 62-20579 discloses an adhesive composition comprising a polymerized resin capable of forming a film, a poly-functional compound having a polyaromatic skeleton, a curing agent capable of initiating the reaction by light or heat, a polyurethane having acrylic groups and bisphenol A-type epoxy resin as photocurable resins, and radical photopolymerization initiator, usable in a process wherein the adhesive is coated on insulated encapsulated wires.
U.S. Pat. No. 4,855,333 proposes a process comprising forming a photocurable adhesive layer in place of prior art thermosetting adhesive layer for wiring, pushing wires into the adhesive layer for wiring, followed by exposing the wiring portion and neighboring portion locally to light so as to cure only the wired portion. According to this process, the Storage Shear Modulus (G') at room temperature is 2 to 4 MPa, the Loss Angle Ratio (R), i.e. Loss shear Modulus (G")/(G') at room temperature is 0.3 to 0.7, and G' heated at lower than 150.degree. C. at the time of wiring is 0.1 MPa or less. The photocurable adhesive used in this process is a radical polymerization type obtained by preliminary reacting bisphenol A type epoxy resin having a molecular weight of 1500 to 5000, a polyfunctional epoxy resin having a molecular weight of 900 to 1500 and acrylic acid, followed by addition of a polyfunctional acrylic resin and a photo initiator.
Japanese Patent Examined Publication No. 1-33958 discloses a process comprising wiring using a photocurable adhesive layer described in Examples of U.S. Pat. No. 4,855,333, and immediately after this, exposing the portion near the wires partially to light for curing.
On the other hand, various processes for curing epoxy resins are known. The following examples relate to cationic photo curing having very close relation to the present invention.
Japanese Patent Unexamined Publication No. 3-252488 discloses a photocurable adhesive composition comprising 100 parts by weight of epoxy resin, 3 to 20 parts by weight by intramolecular epoxy modified polybutadiene, 50 to 300 parts by weight of an inorganic filler, and a cationic photo initiator.
U.S. Pat. Nos. 4,173,551 and 4,275,190 disclose a combination of an aromatic diaryliodonium salt and a copper salt as a cationic polymerization initiator to activate thermosetting action of the initiator.
In order to obtain high density multiple wire wiring boards, the adhesive is required not to use a rubber component having low insulation resistance in order to prevent lowering of insulation resistance, to enhance the positional precision of wires, and to have flexibility, to be able to conduct film formation and to maintain non-sticking properties in order to use previously used manufacturing apparatuses and production processes as much as possible.
In order to prevent the lowering of insulation resistance, for example, Japanese Patent Examined Publication No. 2-12995 discloses an adhesive comprising a polymer component in place of a rubber component and a plasticizer, a solvent or a diluent so as to make it possible to obtain flexibility and film formation. According to this reference, by employing a step of preheating between a wiring step and a press step, the wire swimming is prevented by slightly curing the adhesive layer. But in this preheating step, the viscosity of adhesive layer is temporally lowered to release the stress accumulated on wires, resulting in causing a problem that the wires float and cause misregistration. In order to prevent these problems, when a curing agent having high reactivity, there arise another problems in that the shelf life of adhesive is shortened, and the like.
Further, in the course of improvement of adhesives, there arises another necessity to excellently suppressing the generation of voids in addition to satisfaction of the above-mentioned requirement. That is, on the surface of substrate wired in high density, there is a large relief due to insulated encapsulated wires. Further, at the crossing portions of insulated encapsulated wires, there are many spaces without an adhesive layer. When cured as they are, such spaces remain as voids, resulting in causing short circuit in through holes, lowering resistance to electrolytic corrosion and lowering insulation resistance.
On the other hand, in order to enhance the positional precision of wires, it is necessary to cure the adhesive layer completely immediately after wiring. As such a process, Japanese Patent Examined Publication No. 1-33958 discloses a process wherein a photocurable adhesive layer is used and a wired portion is cured by exposure to light. But, the crossing portions of insulated encapsulated wires cause great relief as mentioned above to generate spaces without the adhesive layer. When cured completely without removing such spaces, the spaces are retained as they are, resulting in causing a great problem, particularly in the case of forming very fine circuits.
According to prior art techniques, there was a problem in that it was difficult to attain both the enhancement of positional precision of wires and removal of voids in order to produce multiple wire wiring boards with high wiring density.
In addition, the use of photocurable adhesives has various problems. In the production of multiple-wire wiring boards, a protective film is peeled from an adhesive layer formed on a substrate and wiring working is sometimes conducted for several hours. When a radical photopolymerization initiator as disclosed in Japanese Patent Unexamined Publication No. 62-20579 is used, since the radical photopolymerization is inhibited by the oxygen in the air at the surface portion of the adhesive layer, the adhesive layer at the surface portion is not cured, resulting in causing a problem of lowering in heat resistance. Further, when a known photocurable adhesive for multiple-wire wiring boards is used, since the glass transition temperature is low, there is a problem in that the reliability as the substrate is low.